U.S. patent number 6,896,035 [Application Number 10/322,943] was granted by the patent office on 2005-05-24 for manufacturing method for continuously cast product of steel.
This patent grant is currently assigned to NKK Corporation. Invention is credited to Yutaka Awajiya, Yukinori Iizuka, Masayuki Nakada, Makoto Suzuki, Koichi Tsutsumi.
United States Patent |
6,896,035 |
Iizuka , et al. |
May 24, 2005 |
Manufacturing method for continuously cast product of steel
Abstract
A manufacturing method for a continuously cast product of steel
includes the steps of detecting a position of crater end of product
by a method for measuring a solidification state of continuously
cast product using a sensor arranged so as to be in non-contact
with the product, and controlling at least one condition selected
from the conditions of the casting speed and the quantity of
secondary cooling water based on the detected position of crater
end. The method for measuring a solidification state of
continuously cast product includes the steps of cooling the product
until a surface layer portion thereof is .alpha. transformed,
transmitting transverse waves of electromagnetic ultrasonic waves
to the cooled product, receiving the signal after the transmitting
signal propagates in the product, and judging the solidification
state of the product based on the received signal.
Inventors: |
Iizuka; Yukinori (Tokyo,
JP), Awajiya; Yutaka (Fukuyama, JP),
Nakada; Masayuki (Fukuyama, JP), Suzuki; Makoto
(Fukuyama, JP), Tsutsumi; Koichi (Fukuyama,
JP) |
Assignee: |
NKK Corporation (Tokyo,
JP)
|
Family
ID: |
26614169 |
Appl.
No.: |
10/322,943 |
Filed: |
December 18, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCTJP0110428 |
Nov 29, 2001 |
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Foreign Application Priority Data
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Apr 25, 2001 [JP] |
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2001-127369 |
Sep 26, 2001 [JP] |
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2001-294017 |
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Current U.S.
Class: |
164/454; 164/413;
164/414; 164/455 |
Current CPC
Class: |
G01N
29/449 (20130101); G01N 29/50 (20130101); G01N
29/343 (20130101); B22D 11/20 (20130101); B22D
11/225 (20130101); B22D 11/16 (20130101); B22D
11/163 (20130101); B22D 11/1206 (20130101); G01N
29/2412 (20130101); G01N 29/348 (20130101); G01N
2291/102 (20130101); G01N 2291/0251 (20130101); G01N
2291/0422 (20130101); G01N 2291/0421 (20130101); G01N
2291/048 (20130101) |
Current International
Class: |
B22D
11/20 (20060101); B22D 11/22 (20060101); B22D
11/16 (20060101); G01N 29/50 (20060101); G01N
29/44 (20060101); G01N 29/34 (20060101); G01N
29/24 (20060101); B22D 011/16 (); B22D 011/20 ();
B22D 011/22 () |
Field of
Search: |
;164/476,477,452,454,455,413,414,150.1,151.2,154.1,154.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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52-130422 |
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Nov 1977 |
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JP |
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52-130422 |
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Nov 1977 |
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JP |
|
53-057088 |
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May 1978 |
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JP |
|
53-106085 |
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Sep 1978 |
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JP |
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53-106085 |
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Sep 1978 |
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JP |
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57-73670 |
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May 1982 |
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JP |
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57-106855 |
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Jul 1982 |
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JP |
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60-100758 |
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Jun 1985 |
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JP |
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61-37356 |
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Feb 1986 |
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JP |
|
62-148850 |
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Jul 1987 |
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JP |
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62-148850 |
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Jul 1987 |
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JP |
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9-174213 |
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Jul 1997 |
|
JP |
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10-197502 |
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Jul 1998 |
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JP |
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11-183449 |
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Jul 1999 |
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JP |
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2000-266730 |
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Sep 2000 |
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JP |
|
Other References
European Search Report for European Application No. EP 01274217.
.
Patent Abstracts of Japan, vol. 011, No. 378 (P-645), abstract of
JP Publication No. 62 148851 A published Jul. 2, 1987. .
Patent Abstracts of Japan, vol. 1998, No. 12, abstract of JP
Publication No. 10 197502 A published Jul. 31, 1998..
|
Primary Examiner: Kerns; Kevin P.
Attorney, Agent or Firm: Frishauf, Holtz, Goodman &
Chick, P.C.
Parent Case Text
This application is a continuation application of International
Application PCT/JP01/10428 (not published in English) filed Nov.
29, 2001.
Claims
What is claimed is:
1. A manufacturing method for a continuously cast product of steel
comprising the steps of: detecting a position of a crater end of
the product by using a method for measuring a solidification state
of the continuously cast product by a sensor arranged so as to be
in non-contact with said product; and controlling at least one
condition selected from the conditions of casting speed and
quantity of secondary cooling water based on said detected position
of the crater end, said method for measuring a solidification state
of the continuously cast product comprising the steps of:
transmitting transverse waves of electromagnetic ultrasonic waves
to said product repeatedly in pulse units as a transmitting signal;
receiving a signal after said transmitting signal propagates in
said product as a receiving signal; and judging the solidification
state of said product by averaging pulses and performing signal
processing of the receiving signal so that the average number is
not less than 16 times and not more than pulse cycles in which
signal intensity after said averaging does not decrease.
2. The manufacturing method for a continuously cast product of
steel according to claim 1, further comprising a step of cooling
said product until a surface layer portion thereof is .alpha.
transformed before the step of transmitting transverse waves of
electromagnetic ultrasonic waves to said product repeatedly in
pulse units as a transmitting signal.
3. The manufacturing method for a continuously cast product of
steel according to claim 2, wherein the solidification state of
said product is measured at a plurality of positions along the
casting direction of said product to detect the position of a
crater end.
4. The manufacturing method for a continuously cast product of
steel according to claim 2, wherein it least one condition selected
from the conditions of the casting speed and the quantity of
secondary cooling water is controlled based on the detected
position of the crater end so that the crater end lies within a
soft reduction zone.
5. The manufacturing method for a continuously cast product of
steel according to claim 2, wherein the solidification state of
said product is measured at a plurality of positions along the
widthwise direction of said product to detect the position of the
crater end.
6. The manufacturing method for a continuously cast product of
steel according to claim 1, wherein the transmitting and receiving
of transverse waves of electromagnetic ultrasonic waves are
performed by using an electromagnet, and an exciting current off
said electromagnet is made a pulse current continuing for a longer
time than a measuring time necessary for the judgment of
solidification.
7. The manufacturing method for a continuously cast product of
steel according to claim 1, wherein the solidification state of
said product is measured at a plurality of positions along the
casting direction of said product to detect the position of a
crater end.
8. The manufacturing method for a continuously cast product off
steel according to claim 1, wherein at least one condition selected
from the conditions of the casting speed and the quantity of
secondary cooling water is controlled based on the detected
position of the crater end so that the crater end lies within a
soft reduction zone.
9. The manufacturing method for a continuously cast product of
steel according to claim 1, wherein the solidification state of
said product is measured at a plurality of positions along the
widthwise direction of said product to detect the position of the
crater end.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a manufacturing method for a
continuously cast product of steel and, more particularly, to a
method for controlling casting conditions by detecting the position
of a solidification end (referred to as a crater end) of a cast
product.
2. Description of Related Arts
In continuous casting of steel, in order to improve the
productivity and quality of product, it is very important that the
position of a crater end of cast product in the casting direction
be detected, and casting conditions be controlled based on the
detection result.
For example, if the casting speed is increased to improve the
productivity, the crater end moves to the downstream side in the
casting direction. However, if the crater end moves beyond a region
in which product support rolls lie, the product is swelled by
static pressure (referred to as bulging), which presents a problem
of degraded quality or casting stop due to large bulging. Also, in
the case where soft reduction is applied to a product to decrease
central segregation of product and thereby achieve high quality, it
is necessary to control the casting speed and the quantity of
secondary cooling water so that the crater end is positioned in a
soft reduction zone. Further, in the case where the position of
crater end is varied greatly in the casting direction by the change
of casting conditions, a product on the upstream side of crater end
in the casting direction solidifies first, and the supply of molten
steel to the downstream side is stopped, so that porosity or
laminar voids are created in the central portion of product, which
causes a defect that greatly decreases the yield of final product.
Also, in the case where the position of crater end varies greatly
in the casting direction, even if the casting speed and the
quantity of secondary cooling water are controlled, it is difficult
to induce the crater end to the soft reduction zone.
In order to detect the position of crater end, it is necessary to
continuously measure the solidification state of product. Various
methods for this measurement have been proposed so far. Among these
methods, many methods in which the transverse waves of ultrasonic
waves (hereinafter referred to as transverse ultrasonic waves) are
utilized have been proposed. This is because since the transverse
ultrasonic wave has a property such that it propagates in a solid
phase only and does not propagate in a liquid phase, if the
transverse ultrasonic waves are transmitted in the thickness
direction at a position of product and a signal indicating that the
transverse ultrasonic waves have propagated in the product is
detected, it can be judged that the position has been solidified
completely, and if the signal is not obtained, it can be judged
that unsolidified layer remains. Also, there is available a method
in which the position of crater end is estimated from time of
flight in which the transverse ultrasonic waves propagates in a
product.
As a method for generating the transverse ultrasonic waves in a hot
product and detecting them, an electromagnetic ultrasonic wave
method in which ultrasonic waves are transmitted and received
electromagnetically is known. As a method for measuring the
solidification state of product by using the electromagnetic
ultrasonic wave method, a method in which a product is held between
two transverse ultrasonic wave sensors and the signal intensity of
transverse ultrasonic waves having propagated in the product is
measured has been disclosed in JP-A-52-130422.
JP-A-62-148850 discloses a method in which an electromagnetic
ultrasonic wave sensor capable of generating longitudinal waves and
transverse waves at the same time is used to measure the
solidification state by the signal intensity of transverse
ultrasonic waves, and the variations in liftoff (gap between
product and sensor) and the abnormality of sensor are checked at
the same time by additionally using a signal of longitudinal
ultrasonic waves propagating in an unsolidified layer.
JP-A-10-197502 discloses a method in which a resonance frequency of
transverse ultrasonic waves in a product is measured, and a solid
phase ratio (ratio of solid phase to solid-liquid coexistence
phase) is determined from this resonance frequency.
However, in these methods for measuring the solidification state of
product by using electromagnetic ultrasonic waves, the sensitivity
is low and the S/N (signal-to-noise ratio) is also low, so that
sufficient measurement accuracy cannot be obtained. Also, for this
reason, the liftoff of electromagnetic ultrasonic wave sensor is
inevitably decreased to about 2 mm, so that continuous measurement
cannot be made stably for a long time.
To solve the problem, for example, JP-A-11-183449 discloses a
method in which a touch roll is installed on the sensor and the
touch roll is pushed against a product to make continuous
measurement for a long time. In this method, however, if the sensor
is used continuously in an environment in which the temperature
exceeds several hundred degrees Centigrade and much scale exists,
the scale gets stuck between the sensor and the product, so that
the sensor may be damaged or the touch roll sticks to the product,
which makes continuous measurement difficult.
Therefore, it is necessary that the liftoff be widened by
increasing the sensitivity of electromagnetic ultrasonic wave
sensor and that the measurement be done in a non-contact manner
without the use of touch roll.
As a method for increasing the sensitivity of electromagnetic
ultrasonic wave sensor, JP-A-53-106085 discloses a method in which
electromagnetic ultrasonic waves by Lorentz's force is used and a
cooling fluid is blown to a hot steel to decrease the temperature
of steel to a temperature not higher than the Curie point, by which
the steel is magnetized and the electric conductivity is increased.
In this method, since the driving force F of electromagnetic
ultrasonic waves by Lorentz's force is expressed by F=B.times.J by
using magnetic flux density B and electric current density J, as B
and J increase, the sensitivity is made higher.
JP-A-2000-266730 discloses a method in which a burst-like
transmitting signal in which at least one selected from frequency,
amplitude, and phase is modulated within a predetermined pulse
width is used, and correlation operation of receiving signal is
performed by using a reference signal of a waveform that is the
same as or similar to the transmitting signal. In this method, the
correlation between receiving signal and transmitting signal is
high, and the correlation between noise and transmitting signal is
low, so that the S/N is increased by the correlation operation.
JP-A-53-57088 discloses a method in which receiving signals are
averaged in synchronization with an electromagnetic ultrasonic wave
generator. In this method, noise has a random waveform generated
for each pulse repetition, so that the S/N is increased by
averaging.
However, in the method described in JP-A-53-106085, the magnetic
flux density of steel is low near the Curie point, so that the
steel must be cooled rapidly to a temperature range 200.degree. C.
or more lower than the Curie point to obtain a high magnetic flux
density, which impairs the quality of product. Also, the conversion
efficiency of electromagnetic ultrasonic waves by Lorentz's force
is very low essentially, so that an effect of increasing the S/N is
little.
If electromagnetic ultrasonic waves are applied to the method
described in JP-A-2000-266730, since the receiving signal is far
weaker than the transmitting signal and the transmitting signal
leaks into the receiving signal, if the pulse width of burst wave
is too long, the transmitting signal hides the receiving signal. In
particular, if this method is applied to continuous casting, the
inside temperature of product changes during the operation, and the
position at which the receiving signal appears varies, so that the
pulse width cannot be made too long, and an effect of increasing
the S/N is little.
If the method described in JP-A-53-57088 is applied to continuous
casting, the position at which the receiving signal appears varies
as described above, so that the average number must be increased.
Therefore, an effect of increasing the S/N is little.
As described above, in the prior art, since the S/N cannot be
increased sufficiently, the liftoff of electromagnetic ultrasonic
wave sensor cannot be increased, so that the position of crater end
cannot be detected stably and exactly in a non-contact state.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a manufacturing
method for a continuously cast product, in which the position of a
crater end is detected stably and exactly in a state of non-contact
with the product, and hence a high-quality product can be
manufactured without a decrease in productivity.
The above object is attained by a manufacturing method for a
continuously cast product of steel comprising the steps of:
detecting a position of crater end of product by using a method (1)
for measuring a solidification state of continuously cast product
by a sensor arranged so as to be in non-contact with the product;
and controlling at least one condition selected from the conditions
of the casting speed and the quantity of secondary cooling water
based on the detected position of crater end, the method (1) for
measuring a solidification state of continuously cast product
comprising the steps of: cooling the product until a surface layer
portion thereof is .alpha. transformed; transmitting transverse
waves of electromagnetic ultrasonic waves to the cooled product as
a transmitting signal; receiving a signal after the transmitting
signal penetrates the product as a receiving signal; and judging
the solidification state of the product based on the receiving
signal.
As the above-described method for measuring a solidification state
of continuously cast product by the sensor arranged so as to be in
non-contact with the product, the following two methods can be
applied in addition to the above-described method (1).
(2) A method comprising the steps of: transmitting transverse waves
of burst-like electromagnetic ultrasonic waves, in which at least
one selected from frequency, amplitude, and phase is modulated
within a pulse width that has a magnitude of 50 to 150% of the
maximum time width not exceeding a time of flight time for
propagating in the product, to the product as a transmitting
signal; receiving a signal after the transmitting signal propagates
in the product as a receiving signal; and judging the
solidification state of the product by performing correlation
operation of the receiving signal by using a reference signal of a
waveform that is the same as or similar to the transmitting
signal.
(3) A method comprising the steps of: transmitting transverse waves
of electromagnetic ultrasonic waves to the product repeatedly in
pulse units as a transmitting signal; receiving a signal after the
transmitting signal propagates in the product as a receiving
signal; and judging the solidification state of the product by
averaging pulses and performing signal processing of receiving
signal so that the average number is not less than 16 times and not
more than pulse cycles in which signal intensity after the
averaging does not decrease.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram schematically showing the relationship between
temperature of steel and sensitivity of electromagnetic ultrasonic
wave sensor;
FIG. 2 is a diagram showing the relationship between transmitting
signal and time of flight;
FIG. 3 is a diagram showing the relationship between time of flight
and averaging;
FIG. 4 is a diagram showing one example of a method for measuring
the solidification state of product, which is an essential
requirement of a method in accordance with the present
invention;
FIG. 5 is a view showing one example of an electromagnetic
ultrasonic wave sensor;
FIG. 6 is a transformation diagram for continuous cooling of steel
subjected to a test;
FIG. 7 is a diagram showing the relationship between average number
and receiving signal amplitude;
FIG. 8 is a diagram showing another example of a method for
measuring the solidification state of product, which is an
essential requirement of a method in accordance with the present
invention; and
FIG. 9 is a view showing one example of an embodiment of a method
in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The inventors conducted studies on the sensitivity of a sensor at
the time when an electromagnetic ultrasonic wave sensor is arranged
so as not to be in contact with a product being continuously cast
and the transverse waves of electromagnetic ultrasonic waves are
transmitted and received. As a result, the inventors found that a
high S/N can be obtained by three methods described below.
(1) First Method
A first method is one in which a product is cooled until the
surface layer portion is .alpha. transformed, the transverse waves
of electromagnetic ultrasonic waves are transmitted as a
transmitting signal to a portion .alpha. transformed by cooling by
a transmitting sensor, and a signal after the transmitting signal
propagates in the product is received as a receiving signal by a
receiving sensor.
The first method differs from the method described in
JP-A-53-106085 in that the product surface layer portion is .alpha.
transformed by cooling and that the transverse waves of
electromagnetic ultrasonic waves are used. In the method described
in JP-A-53-106085, in the case where the cooling rate is high and
the cooling time is short at the time when the product is cooled,
even if the temperature of product is decreased to a value not
higher than the Curie point, a non-magnetic .gamma. phase remains,
and the magnetism does not recover instantly. On the other hand, in
the first method, since the product is cooled until the surface
layer portion is .alpha. transformed so that the magnetism recovers
surely, as an electromagnetic ultrasonic wave generating mechanism,
in addition to the effect of Lorentz's force, the effect of
magnetostriction is dominant, and thus a high S/N can be obtained.
Specifically, as shown in FIG. 1, at a temperature higher than the
Curie point, electromagnetic ultrasonic waves are generated by only
the effect of Lorentz's force. However, if the steel is cooled, the
magnetism recovers and the effect of magnetostriction is added due
to supercooling at a temperature slightly lower than the Curie
point. Although the effect of Lorentz's force is increased by
cooling, the effect of magnetostriction is far greater than the
effect of Lorentz's force, so that the sensitivity of
electromagnetic ultrasonic waves is increased, and thus a high S/N
can be obtained.
As the product surface layer portion .alpha. transformed by
cooling, a propagation depth in which the depth from the surface
depends on the frequency of electromagnetic ultrasonic waves is
preferable. For example, when the frequency is 1 MHz, a portion to
a depth greater than about 0.1 to 0.2 mm has only to be .alpha.
transformed. Although it is preferable that the whole structure be
made .alpha. phase completely by transformation, since the way of
.alpha. transformation in the surface layer portion is changed by
the cooling condition and composition etc. of steel, even if the
.gamma. phase remains partially, the effect of the first method can
be obtained.
(2) Second Method
A second method is one in which the transverse waves of burst-like
electromagnetic ultrasonic waves having the maximum pulse width in
which the time of flight is not exceeded, that is, the transverse
waves of burst-like electromagnetic ultrasonic waves in which at
least one selected from frequency, amplitude, and phase is
modulated within a predetermined pulse width set near the maximum
time width determined from product thickness, product temperature,
and sound velocity are transmitted by a transmitting sensor, the
signal after the transmitting signal propagates in the product is
received as a receiving signal by a receiving sensor, and
correlation operation of receiving signal is performed by using a
reference signal of a waveform that is the same as or similar to
the transmitting signal.
In the second method, like the method described in
JP-A-2000-266730, the correlation operation of receiving signal is
performed by using a modulated transmitting signal. The second
method is characterized in that the pulse width of transmitting
signal is set near the maximum time width determined from product
thickness, product temperature, and sound velocity.
As shown in FIG. 2, the receiving signal having propagated in the
product appears at a position lagging behind the transmitting
signal by time of flight. The time of flight dT can be estimated
from the following equation (1) by using product thickness d,
product temperature T(x), sound velocity C(T), and average product
temperature Ta. ##EQU1##
Therefore, if the product thickness d and the average product
temperature Ta according to the operation conditions are determined
beforehand, the time of flight dT can be determined. By setting the
maximum pulse width in the range in which this time of flight is
not exceeded, the sensitivity of electromagnetic ultrasonic waves
is increased, and thus a high S/N can be obtained.
(3) Third Method
A third method is one in which the transverse waves of
electromagnetic ultrasonic waves are transmitted repeatedly in
pulse units as a transmitting signal by a transmitting sensor, a
signal after the transmitting signal propagates in the product is
received as a receiving signal by a receiving sensor, the pulses
are averaged, and signal processing of receiving signal is
performed so that the average number is not less than 16 times and
not more than the pulse cycles in which the signal intensity after
averaging does not decrease, that is, the change of pulse width
caused by the change of signal propagation time is not more than
pulse cycles corresponding to measurement time in which the signal
intensity is not decreased by pulse addition, preferably 256 times.
Specifically, this method is one in which averaging is performed in
number of times in which the average number is not less than 16
times and the signal intensity after averaging does not decrease,
and the receiving signal is processed in synchronization with the
transmitting signal.
If synchronous averaging is performed as in the method described in
JP-A-53-57088, since the product temperature changes depending on
the operation state, as shown in FIG. 3, the position at which the
receiving signal appears changes every moment. Therefore, when the
change rate of time of flight is high, the receiving signal after
averaging decreases. FIG. 3 shows the case where the average number
is two times. If the average number is further increased, the
change rate of time of flight further increases, and the receiving
signal is further made small.
In the third method, in order to avoid this problem, the average
number is determined as described below.
Taking the receiving signal as a sinusoidal wave with a frequency
f, the change rate of time of flight per unit time of the receiving
signal as .tau.[=(t2-t1)/T], the pulse repetition frequency as PRF
(=1/Tprf), and the average number as Na, the amplitude Xs of
receiving signal after averaging is expressed by the following
equation (2). ##EQU2##
Also, the amplitude Xn of noise is expressed by the following
equation (3)
Therefore, the improvement amount P of S/N after averaging, which
can be expressed by the following equation (4), is obtained.
Therefore, the maximum average number is determined based on these
equations, and the average number is set so that it does not exceed
the maximum number of times, by which a high S/N can be
obtained.
If the solidification state of product is judged by the
above-described first, second, or third method, by which a high S/N
can be obtained, the position of crater end can be detected exactly
by a non-contact method. Therefore, if at least one condition
selected from the conditions of the casting speed of continuous
casting and the quantity of secondary cooling water is controlled
based on the detected position of crater end, a high-quality
product can be manufactured without a decrease in productivity. For
example, if at least one condition selected from the conditions of
the casting speed of continuous casting and the quantity of
secondary cooling water is controlled based on the detected
position of crater end and the crater end is caused to lie in the
soft reduction zone, the central segregation at the time of
solidification can be decreased, so that a high-quality product can
be manufactured.
The improvement in S/N achieved by the above-described first,
second, or third method is based on a different principle.
Therefore, if these methods are combined, a higher S/N can be
obtained.
If by using the above-described first, second, or third method or a
method in which these methods are combined, the transverse waves of
electromagnetic ultrasonic waves are transmitted and received by
using an electromagnet, and the excited current of electromagnet is
used as a pulse current continuing for a longer time than the
measurement time necessary for solidification judgment, a further
higher S/N can be obtained. Therefore, since a sufficient liftoff
can be provided, a continuously cast product can be manufactured
more stably.
In order to detect the position of crater end from the measurement
result of solidification state, as described above, there are two
methods: a method for detecting the crater end position by the time
of flight of receiving signal and a method for detecting the crater
end position from the presence of receiving signal.
In the former method, a phenomenon that the sound velocity of
ultrasonic waves depends on the temperature, that is, a phenomenon
that the higher the temperature in the product is, the longer the
time of flight of ultrasonic waves is utilized. If a set of sensors
is surely provided at a position of product on the downstream side
of crater end to measure the time of flight of ultrasonic waves,
when the crater end comes close to the sensors and the temperature
in the product rises, the time of flight becomes long. Therefore,
if the relationship between the position of crater end and the time
of flight is determined beforehand by a riveting test or by a
method in which a plurality of sensors are provided in the casting
direction, the position of crater end can be detected.
In the latter method, after the transmitting signal is transmitted,
a gate is provided in the time zone in which the receiving signal
appears, the maximum value in the gate is determined to determine
the intensity of receiving signal, and the presence of receiving
signal is judged by checking whether or not the intensity exceeds a
certain threshold value, by which the position of crater end is
detected. In this method, it is necessary to provide sensors at a
plurality of positions along the casting direction of product.
In either of these detecting methods, a receiving signal with a
high S/N is obtained by the above-described first, second, or third
method or a solidification state measuring method in which these
methods are combined, and therefore the time of flight can also be
measured with high accuracy, so that the position of crater end can
be found exactly.
If the measurement of solidification state is made at a plurality
of positions along the widthwise direction of product by the
above-described first, second, or third method or a method in which
these method are combined, a widthwise profile of crater end is
found. If the quantity of secondary cooling water in the widthwise
direction is controlled based on the profile, the uniformity of
widthwise profile can be achieved, by which a high-quality product
with less segregation can be obtained. The measurement at a
plurality of positions along the widthwise direction can be made by
scanning a set of sensors in the widthwise direction or by
providing a plurality of sets of sensors in the widthwise
direction.
The position of crater end is detected by the above-described
first, second, or third method or a method in which these method
are combined, and based on the detected position of crater end, a
soft reduction zone for applying soft reduction to the product can
be provided, or the product can also be cut.
As a solidification state measuring apparatus for a continuously
cast product of steel, which judges the solidification state of
product by the above-described first, second, or third method or a
method in which these method are combined, the following
apparatuses can be applied:
(i) A solidification state measuring apparatus for a continuously
cast product of steel having a cooling device for cooling the
product until the surface layer portion of product is .alpha.
transformed, a transmitting electromagnetic ultrasonic wave sensor
for transmitting the transverse waves of electromagnetic ultrasonic
waves to the cooled product as a transmitting signal, a receiving
electromagnetic ultrasonic wave sensor for receiving the signal
after the transmitting signal penetrates the product as a receiving
signal, and an evaluating section for judging the solidification
state of product based on the receiving signal.
(ii) A solidification state measuring apparatus for a continuously
cast product of steel having a transmitting electromagnetic
ultrasonic wave sensor for transmitting the transverse waves of
burst-like electromagnetic ultrasonic waves, in which at least one
selected from frequency, amplitude, and phase is modulated within a
pulse width that has a magnitude of 50 to 150% of the maximum time
width not exceeding the time of flight for propagating in the
product, to the product as a transmitting signal, a receiving
electromagnetic ultrasonic wave sensor for receiving the signal
after the transmitting signal propagates in the product as a
receiving signal, a correlation processing section for performing
correlation operation of receiving signal by using a reference
signal of a waveform that is the same as or similar to the
transmitting signal, and an evaluating section for judging the
solidification state of product based on the operation result.
(iii) A solidification state measuring apparatus for a continuously
cast product of steel having a transmitting electromagnetic
ultrasonic wave sensor for transmitting the transverse waves of
electromagnetic ultrasonic waves repeatedly in pulse units as a
transmitting signal, a receiving electromagnetic ultrasonic wave
sensor for receiving the signal after the transmitting signal
propagates in the product as a receiving signal, a synchronous
averaging section for averaging the pulses and performing signal
processing of receiving signal so that the average number is not
less than 16 times and not more than the pulse cycles in which the
signal intensity after averaging does not decrease, and an
evaluating section for judging the solidification state of product
based on the result of signal processing.
If the above-described solidification state measuring apparatuses
of items (i) to (iii) are combined, a higher S/N can be
obtained.
Embodiment 1
FIG. 4 shows one example of a method for measuring the
solidification state of product, which is an essential requirement
of a method in accordance with the present invention.
A continuously cast product 1 of carbon steel moves to the right
side in the figure by support rolls 2. An unsolidified portion 7
exists within the product 1, and the tip end thereof is a crater
end. A surface layer portion 6 of the product 1 is cooled by water
cooling nozzles 5 provided between the rolls 2, and is transformed
from .gamma. phase to .alpha. phase. A transmitting electromagnetic
ultrasonic wave sensor 3 and a receiving electromagnetic ultrasonic
wave sensor 4 are arranged face to face so as to hold the .alpha.
transformed portion therebetween.
A transmitting output system for outputting the transverse waves of
electromagnetic ultrasonic waves from the transmitting
electromagnetic ultrasonic wave sensor 3 as a transmitting signal
is composed of a trigger signal generating section 8 for
transmitting signal, a transmitting signal generating section 9,
and a pulse width setting section 16 for setting the pulse width of
burst waves. Further, the transmitting signal generating section 9
is composed of a burst wave generating section 14 for generating
burst waves of a pulse width set based on the trigger signal and a
power amplifying section 15 for amplifying the generated burst
waves and outputting them to the transmitting electromagnetic
ultrasonic wave sensor 3 as a transmitting signal.
A receiving processing system for receiving the signal after
propagating in the product by means of the receiving
electromagnetic ultrasonic wave sensor 4 and processing the signal
is composed of a receiving signal amplifying section 10, a
synchronous averaging section 12, a setting section for average
number 13, a correlation processing section 17, and an evaluating
section 11 for judging the solidification state from the receiving
signal.
When a transmitting timing signal is generated from the trigger
signal generating section 8, the burst wave generating section 14
generates a burst-like transmitting signal in which at least one
selected from frequency, amplitude, and phase is modulated. Herein,
a pulse width is specified by the pulse width setting section 16.
The transmitting signal is amplified by the power amplifying
section 15, and is applied to the transmitting electromagnetic
ultrasonic-wave sensor 3.
As shown in FIG. 5, in the .alpha. transformed surface layer
portion 6 of the product 1, a high-frequency oscillating magnetic
field By caused by the transmitting signal is applied in parallel
to the surface of the product 1 by a coil 19 provided in the
transmitting electromagnetic ultrasonic wave sensor 3. As a result,
since a stress is applied in parallel to the surface of the product
1 by magnetostriction, shear waves or transverse waves are
generated. The transmitting electromagnetic ultrasonic wave sensor
3 is provided with a magnet 20 having magnet poles in the vertical
direction to increase the effect of magnetostriction by the static
magnetic field Bs. This magnet may be a permanent magnet or an
electromagnet.
On the opposite surface of the product 1, the receiving
electromagnetic ultrasonic wave sensor 4 constructed as shown in
FIG. 5 is arranged face to face, and a static magnet field Bs is
applied to the .alpha. transformed surface layer portion 6 of the
product 1 by a magnet. When the transverse waves of electromagnetic
ultrasonic waves propagate in the product and reach this portion,
the magnetic permeability of this portion is changed by the reverse
effect of magnetostriction. As a result, the magnetic flux Bs
crossing a coil of the receiving electromagnetic ultrasonic wave
sensor 4 changes, so that a voltage is generated in the coil by
electromagnetic induction, by which the receiving signal can be
obtained.
This receiving signal is amplified by the amplifying section 10,
and then is sent to the synchronous averaging section 12, where the
receiving signal is averaged by the number of times set by the
setting portion for average number 13. In the synchronous averaging
section 12, although averaging can be performed by various methods,
in this embodiment, the signal is expressed by numbers by
performing A/D conversion, and is averaged by a computer in
synchronization with the signal of the trigger generating section
8. The averaging can be performed by using the following equation
(5). ##EQU3##
where, Xi(j) is input signal, Yi(j) is output signal, Na is the
average number, i is pulse repetition, and
-.infin..ltoreq.i.ltoreq..infin., 0.ltoreq.j.ltoreq.n-1 (n: number
of data for one flaw detection signal).
The averaged receiving signal is sent to the correlation processing
section 17, where the signal is subjected to correlation operation
by the following equation (6). If the calculation is not performed
by using the equation (6), but input signal subjected to FFT (fast
Fourier transform) and conjugate of reference signal subjected to
FFT are multiplied together, and the result is subjected to inverse
FFT and is outputted, processing can be performed at the highest
speed. ##EQU4##
where, Xi(j) is input signal, Yi(j) is output signal, C(j) is
reference signal, Nc is the number of reference signals, i is pulse
repetition, and -.infin..ltoreq.i.ltoreq..infin.,
0.ltoreq.j.ltoreq.n-1 (n: number of data for one flaw detection
signal).
The burst wave generating section 14 generates the burst-like
transmitting signal in which at least one selected from frequency,
amplitude, and phase is modulated. As one example of modulation
system, frequency modulated chirp waves are shown by the following
equation (7).
where, fc is central frequency of chirp waves, Bw is a frequency
sweep width of chirp waves, Tw is pulse width of chirp waves, and
0.ltoreq.t.ltoreq.Tw.
Since the waveform of receiving signal having propagated in the
product 1 is analogous to that of the transmitting signal, the
receiving signal having passed through the correlation processing
section 17 is caused to have a sharp waveform with a short pulse
width by the pulse compression effect that the pulse width of
receiving signal is made shorter than that of transmitting signal.
This is useful in the following two respects when the
solidification state is judged by using the evaluating section 11
successively. The first respect: when the intensity of receiving
signal is determined, a gate has only to be provided in the time
zone of receiving signal to determine the maximum value in the
gate. In the case of a sharp waveform with a short pulse width,
excess noise is not picked up. The second respect: when the time of
flight of receiving signal is determined, a pulse with a short
width increases the time accuracy, so that the solidification state
can be judged with high accuracy.
Thus, the solidification state at a measurement position of the
product 1 is measured by obtaining the output from the correlation
processing section 17 and then judging the solidification state in
the evaluating section 11.
The processing in the synchronous averaging section 12, the
correlation processing section 17, and the evaluating section 11
can be performed by using one or more computers. Also, in the
processing in the evaluating section 11, the presence of receiving
signal can be detected by the use of a comparator or by manual
means.
Improvement examples of S/N will be described below.
1. In the case where the above-described first method is used
A product having a transformation diagram for continuous cooling
shown in FIG. 6, which moves at a speed of 40 mm/s and has a
surface temperature of 900.degree. C., was cooled at a cooling rate
of -20.degree. C./s by the water cooling nozzles 5 disposed in
front of the transmitting electromagnetic ultrasonic wave sensor 3,
and the S/N of receiving signal was measured. As seen from FIG. 6,
the transformation start temperature (curve a) at the time when the
product is cooled at a cooling rate of -20.degree. C./s is about
620.degree. C. The effect of magnetostriction as described by using
FIG. 1 increases as the product is cooled to a temperature lower
than the transformation end temperature (curve b). In this example,
the product was cooled so that the surface temperature thereof is
600.degree. C. to prevent surface cracking etc. Therefore, since a
temperature decrease of 300.degree. C. is needed, the length of a
cooling zone was set to 300/20.times.40=600 mm.
As a result, the S/N was improved by 10 dB as compared with the
case where such cooling is not performed.
2. In the case where the above-described second method is used
As described above, the receiving signal is received a time of
flight late after transmitting, so that the pulse width of
transmitting signal must be shorter than the time of flight to
prevent the leak of transmitting signal from overlapping with the
receiving signal. Also, the time of flight is determined from
product thickness, product temperature, and sound velocity, and the
sound velocity depends on product temperature T and steel type,
being approximately 3000-0.65.times.T m/s in the case of transverse
waves and carbon steel. Therefore, with decreasing temperature, the
sound velocity is high, and the time of flight is short.
Thereupon, at a measurement position, since the time of flight is
the shortest when the product temperature is the lowest, the time
of flight at this time is the maximum time width. Therefore, the
pulse width of transmitting signal should be set to a vicinity of
this value. In the present invention, since the solidification
state of product must be judged, the case where the product
temperature is the lowest is thought to be the case where the
center temperature of product is about 1100.degree. C. and the
average temperature thereof is about 1000.degree. C.
Since the S/N is approximately proportional to the square root of
pulse width, if the pulse width is decreased to about 1/2, the S/N
decreases by about 6 dB and the effect becomes little. On the other
hand, if the pulse width is too long, overlapping with the
receiving signal occurs. At both ends of waveform, the amplitude is
somewhat decreased according to the characteristics of sensor and
amplifier, so that a pulse width up to 1.5 times of the maximum
time width is allowable. Therefore, the range of pulse width in
which the S/N can be improved is 50 to 150% of the maximum time
width, preferably 80 to 120% thereof.
Table 1 gives the optimal pulse width of transmitting signal
determined for products having a thickness of 200 mm, 250 mm, and
300 mm. Herein, the lowest average temperature was set at
1000.degree. C., and the sound velocity at that time was set at
2350 m/s.
TABLE 1 Product thickness 200 mm 250 mm 300 mm Min. time of flight
85 .mu.s 106 .mu.s 128 .mu.s Pulse width 68-102 .mu.s 85-128 .mu.s
102-153 .mu.s
When the leakage signal of transmitting signal is large, a
receiving amplifier is saturated by the leakage signal, so that a
time dead zone is sometimes produced by a so-called run-in
phenomenon. Therefore, when there is a run-in phenomenon, the pulse
width has only to be set at a value obtained by subtracting run-in
time from the value given in Table 1.
The S/N of receiving signal was actually measured in the case where
the thickness is 250 mm, the pulse width is 100 .mu.m, and the
frequency is 100 kHz. As a result, the S/N increased by 12 dB as
compared with one sinusoidal wave of 100 kHz. Therefore, by setting
the pulse width in the aforementioned range, the S/N increases by
at least 6 dB.
3. In the case where the above-described third method is used
As described above, as the change rate of time of flight due to
temperature change during casting increases, the receiving signal
may be made small by averaging. According to the study on the
change rate of time of flight, the maximum change rate was about
0.03 to 0.3 .mu.s s/s. Therefore, taking this value as a parameter,
and taking the frequency of ultrasonic waves as 100 kHz, and the
pulse repetition frequency as 100 Hz, the relationship between
average number and amplitude of receiving signal was determined
based on the equation (2).
As a result, as shown in FIG. 7, when the change rate .tau. of time
of flight per unit time is the highest of 0.3 .mu.s/s, a decrease
in amplitude scarcely occurs if the average number is less than 256
times. Therefore, 256 was specified to be the maximum average
number.
At this time, the improvement amount P of S/N shown in the equation
(4) is 24 dB. In calculating the maximum average number, a prior
condition is that if the decrease in receiving signal intensity is
about 1 dB, the amplitude scarcely decreases. Specifically, the
maximum average number when the change rate .tau. of time of flight
is 0.3 .mu.s/s is 256 times and the decrease in receiving signal
intensity is 1 dB. Even in the case of other change rates .tau. of
time of flight, the maximum average number is calculated in the
same way.
Since the less the average number is, the smaller the improvement
amount of S/N is, 16 times or more is preferable. In this case, an
improvement amount of +12 dB can be obtained. Inversely, if the
average number is too large, the amplitude decreases as shown in
FIG. 3, so that it is preferable that the average number be within
about two times of the above-described maximum average number. The
optimal average number is 50 to 200% of the maximum average
number.
As is apparent from the equation (2), in the case where the
frequency of ultrasonic waves or the pulse repetition frequency is
changed, if .tau. is changed proportionally in FIG. 7, the
relationship between average number and amplitude of receiving
signal can be determined.
4. In the case where the first, second, and third methods are
combined
Since the first, second, and third methods are based on different
principles as described above, a combination of all of these
methods increases the S/N by 10+6+12=28 dB. Also, since the liftoff
sensitivity characteristic of electromagnetic ultrasonic wave
sensor is about -4 dB/mm, the liftoff can be widened by 28/4=+7
mm.
Even if all of these methods are not combined, for example, by a
method in which the first and second methods are combined, the S/N
can be increased by 10+6=16 dB, and the liftoff can be widened by
16/4=+4 mm. Therefore, non-contact measurement can be made surely
as compared with the conventional case where the liftoff is 1 to 2
mm.
Similarly, by a method in which the first and third methods are
combined, by a method in which the second and third methods are
combined, or by the third method in which the average number is 64
times or more, the liftoff can be widened by about +5.5 mm, +4.5
mm, or +4.5 mm, respectively.
Embodiment 2
FIG. 8 shows another example of a method for measuring the
solidification state of product, which is an essential requirement
of a method in accordance with the present invention.
In FIG. 8, a pulse magnetizing current generating section 18 for
generating a pulse magnetizing current with a high peak-to-peak
value to increase a magnetizing force is added to the configuration
shown in FIG. 4.
The pulse magnetizing current generating section 18 generates a
pulse magnetizing current in synchronization with the signal of the
trigger signal generating section 8. The duration of pulse
magnetizing current should be a time relating to the ultrasonic
wave measurement, and is suitably not shorter than about two times
of time of flight, that is, not shorter than 200 .mu.s. If the
duration of pulse magnetizing current is at this level, the time
with respect to the repetition frequency of transmitting pulse is
about 1/50, so that the calorific value due to magnetizing current
is very low, and thus a high current can be carried. Therefore,
when a d.c. current was used, the magnetizing current had a limit
of about 3 A, but when a pulse magnetizing current was used, a
peak-to-peak value of 10 A could be obtained, so that the S/N
increased by about 10 dB.
If a pulse magnetizing current is applied to the above-described
first, second, or third method, the liftoff can be widened by +5
mm, +4 mm, or +5 mm, respectively.
Even in the case where a d.c. current is used, if the following
method (1) or (2) is used, the magnetizing current can be increased
to about 10 A, and thus the same effect as that in the case where a
pulse magnetizing current is used can be achieved.
(1): The resistance of copper wire is reduced by increasing the
diameter of copper wire used for an electromagnet.
(2): The cooling capacity is increased by speeding up the
circulation of a cooling medium used for cooling an
electromagnet.
Embodiment 3
FIG. 9 shows one example of an embodiment of a method in accordance
with the present invention.
A continuous casting machine 31 is provided with a mold 22 for
pouring and solidifying molten steel, and under the mold 22, a
plurality of product support rolls 2 are arranged face to face. On
the downstream side of the product support rolls 2, a plurality of
conveying rolls 29 and a gas cutter 30 operated in synchronization
with the casting speed of a product 1 are provided. For the product
support rolls 2, there is provided a secondary cooling zone 23
consisting of first cooling zones 24a, 24b, second cooling zones
25a, 25b, third cooling zones 26a, 26b, and fourth cooling zones
27a, 27b from a position just under the mold 22 toward the
downstream side.
In each cooling zone of the secondary cooling zone 23, secondary
cooling water is sprayed from a plurality of spray nozzles for air
mist spray or for water spray to the surface of the product 1.
Some of the product support rolls 2 are provided so that the
distance between the rolls arranged face to face decreases
gradually toward the downstream side in the casting direction of
the product 1, by which a soft reduction zone 28 capable of
providing a reduction force to the product 1 is formed. The roll
distance of the product support rolls 2 can be changed during
casting by remote control using hydraulic pressure or an electric
motor, so that the soft reduction zone 28 can be provided anywhere,
for example, in a curved portion. In other words, the soft
reduction zone 28 can be moved to any portion in the casting
direction according to a crater end 7a of the product 1.
In this soft reduction zone 28, the soft reduction rate of the
product 1 is set at 0.6 to 1.5 mm/min. If the soft reduction rate
is lower than 0.6 mm/min, an effect of reducing segregation is
little, and on the other hand, if the soft reduction rate exceeds
1.5 mm/min, molten steel is squeezed out in the direction opposite
to the casting direction, so that negative segregation is produced
in a part of product central portion. Also, the total reduction is
set at 2 to 6 mm.
Between the product support rolls 2 on the downstream side of the
secondary cooling zone 23, transmitting electromagnetic ultrasonic
wave sensors 3, 3a and 3b, and receiving electromagnetic ultrasonic
wave sensors 4, 4a and 4b are provided at three points in the
casting direction to detect the position of the crater end 7a of
the product 1.
In the continuous casting machine 31 constructed as described
above, the manufacturing method for a continuously cast product of
steel in accordance with the present invention is carried out as
described below.
Molten steel poured into the mold 22 through an immersion nozzle is
formed into the product 1 having a solidified shell 21 cooled by
the mold 22 and an inside unsolidified layer 7, and is continuously
drawn to the downside while being supported by the product support
rolls 2. The product 1 is cooled by the secondary cooling zone 23
during the time when the product 1 passes through the product
support rolls 2, so that the thickness of the solidified shell 21
increases, and finally solidification proceeds to the product
central portion. The completely solidified end in the casting
direction is the crater end 7a.
The solidification state is measured by the above-described various
methods for measuring the solidification state of product by using
the transmitting and receiving electromagnetic ultrasonic wave
sensors 3, 3a, 3b. 4, 4a and 4b, by which the position of the
crater end 7a can be detected exactly.
If soft reduction is applied to a product by the following method
M1 or M2 based on the detected position of the crater end 7a, the
central segregation of the product 1 can be reduced.
(1) Method M1: When a signal sent from the transmitting
electromagnetic ultrasonic wave sensor 3 is detected by the
receiving electromagnetic ultrasonic wave sensor 4, the casting
speed is increased, or the quantity of secondary cooling water is
decreased, by which the position of the crater end 7a is moved to
the downstream side in the casting direction. On the other hand,
when a signal sent from the transmitting electromagnetic ultrasonic
wave sensor 3a is not detected by the receiving electromagnetic
ultrasonic wave sensor 4a, the casting speed is decreased, or the
quantity of secondary cooling water is increased, by which the
position of the crater end 7a is moved to the upstream side in the
casting direction. Thus, the crater end 7a is caused to lie within
a fixed soft reduction zone 28, whereby the product 1 having less
central segregation can be obtained.
(2) Method M2: The position of the soft reduction zone 28 is
changed by remote control according to the detected position of the
crater end 7a so that the crater end 7a lies within the soft
reduction zone 28. Thus, most of the crater end 7a of the product 1
including an unsteady region can be caused to lie within the soft
reduction zone 28, so that the product 1 having less central
segregation can be obtained throughout from casting start to
end.
In order to maximize the productivity of the continuous casting
machine 31, the crater end 7a must be moved to a position on the
downstream side of the continuous casting machine 31. Therefore,
when a signal sent from the transmitting electromagnetic ultrasonic
wave sensor 3a is detected by the receiving electromagnetic
ultrasonic wave sensor 4a, the casting speed is increased, or the
quantity of secondary cooling water is decreased, by which the
position of the crater end 7a is moved to the downstream side in
the casting direction. On the other hand, when a signal sent from
the transmitting electromagnetic ultrasonic wave sensor 3b is not
detected by the receiving electromagnetic ultrasonic wave sensor
4b, the casting speed is decreased, or the quantity of secondary
cooling water is increased, by which the position of the crater end
7a is moved to the upstream side in the casting direction. Thus,
the crater end 7a can be moved to the downstream side of the
continuous casting machine 31.
When the crater end 7a is detected continuously, if the position of
the crater end 7a changes greatly in the casting direction, that
location is stored in a computer, and that location is compared
with the position to be cut by the gas cutter 30. When that
location corresponds to the position to be cut, the product 1 is
cut at a position except that location by the gas cutter 30 to
obtain a product 1a. By doing this, porosity and laminar voids
produced in the central portion of the product 1 do not appear on
the cut surface, and are pressed at following hot rolling, so that
decrease in yield due to the porosity and laminar voids can be
prevented. If the change rate of the position of the crater end 7a
in the casting direction is 0.5 m/min or higher, cutting should be
performed at a position at least 1 m separated from that
location.
As described above, the position of the crater end 7a can be
detected by using the time of flight measured by the
electromagnetic ultrasonic wave sensor. Therefore, the casting
speed and the quantity of secondary cooling water are regulated
depending on the time of flight, whereby the crater end 7a can be
maintained at a predetermined position, for example, in the soft
reduction zone.
* * * * *